A three-phase voltage inverter is used for an inverter for, for example, an inverter for controlling to drive a motor of a refrigeration air conditioning apparatus. To facilitate understanding of the present invention, a constitution and an operation of a conventional inverter are explained below with reference to FIGS. 36 to 42.
FIG. 36 is a block diagram of an example of the conventional inverter. The conventional inverter shown in FIG. 36 includes an inverter main circuit 1 and an inverter control unit 2 that generates a three-phase PWM signal that is a drive signal for a semiconductor switching element included in the inverter main circuit 1. In other words, the present invention relates to an improvement of the inverter control unit 2.
The inverter main circuit 1 is a well-known circuit including a DC power supply 3 that gives a bus voltage Vdc, three sets of semiconductor switching elements (5a, 5b), (5c, 5d), and (5e, 5f) that are connected in series between a DC bus 4a connected to a positive pole of the DC power supply 3 and a DC bus 4b connected to a negative pole of the DC power supply 3, and flywheel diodes 6a to 6f that are connected in series to the respective semiconductor switching elements. A motor 7 is connected to respective DC connection terminals of the three sets of semiconductor switching elements (5a, 5b), (5c, 5d), and (5e, 5f).
A DC current detecting unit 9, which detects a DC bus current Idc used in the inverter control unit 2, is provided in, for example, the DC bus 4b. The DC current detecting unit 9 includes a detection element (a resistor, a current transformer, etc.) inserted in the DC bus 4b and an amplifier that amplifies a both-end voltage of the detection element (the resistor) or an output voltage of the detection element (the current transformer). The DC current detecting unit 9 converts an output voltage of this amplifier into an electric current to obtain a DC bus current Idc.
The inverter control unit 2 includes a phase-current discriminating unit 11 that discriminates phase currents Iu, Iv, and Iw from the DC bus current Idc inputted from the DC current detecting unit 9, a unit 12 that calculates an excitation current and a torque current for calculating an excitation current Iγ (a γ axis current) and a torque current Iδ (a δ axis current) from the phase currents Iu, Iv, and Iw, a voltage-command-vector calculating unit 13 that calculates a voltage command vector V* used in the next control from the excitation current Iγ and the torque current Iδ, a PWM-signal creating unit 14 that produces conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn, which are three-phase PWM signals during one carrier period, from the voltage command vector V*, and a PWM-signal generating unit 15 that generates driving signals Up, Un, Vp, Vn, Wp, and Wn, which are three-phase PWM signals applied to the semiconductor switching elements 5a to 5f, from the conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn. Note that a subscript “p” means a positive pole side and a subscript “n” means a negative pole side.
An operation of the inverter control unit 2 is explained. Among the semiconductor switching elements 5a to 5f of the inverter main circuit 1, the semiconductor switching elements 5a, 5c, and 5e connected to the DC bus 4a on the positive pole side perform an ON operation or the semiconductor switching elements 5b, 5d, and 5f connected to the DC bus 4b on the negative pole side perform an ON operation. Since there are the semiconductor switching elements for three phases, eight (23=8) kinds of switching patterns or switching modes are present in total. These are states of output to the motor 7.
As state representation of the semiconductor switching elements, an ON operation state of the semiconductor switching elements is represented as a logical value 1 and an OFF operation state of the semiconductor switching elements is represented as a logical value 0. Eight kinds of states of output to the motor 7 are associated with eight kinds of voltage vectors (basic voltage vectors) of V0 to V7 as described below. Among these eight kinds of voltage vectors, V1 to V6 are voltage vectors corresponding to six switching modes having vector lengths and the remaining V0 and V7 are voltage vectors corresponding to two switching modes not having vector lengths. The voltage vectors V0 and V7 are specifically referred to as “zero vectors”. The voltage vectors V1 to V6 are often referred to as “basic voltage vectors” to be distinguished from the “zero vectors”.
A correspondence relation among the voltage vectors V1 to V6 is represented by logical states of the switching elements connected to the DC bus 4a, namely, a logical state of W-phase positive pole side switching elements, a logical state of V-phase positive pole side switching elements, and a logical state of U-phase positive pole side switching elements. The logical states (0, 0, 1) correspond to the voltage vector V1, the logical states (0, 1, 0) correspond to the voltage vector V2, the logical states (0, 1, 1) correspond to the voltage vector V3, the logical states (1, 0, 0) correspond to the voltage vector V4, the logical states (1, 0, 1) correspond to the voltage vector V5, and the logical states (1, 1, 0) correspond to the voltage vector V6.
A correspondence relation between the two zero vectors V0 and V7 is represented by logical states of the switching elements connected to the DC bus 4a, namely, a logical state of W-phase positive pole side switching elements, a logical state of V-phase positive pole side switching elements, and a logical state of U-phase positive pole side switching elements. The logical state (0, 0, 0) corresponds to the zero vector V0 and the logical state (1, 1, 1) corresponds to the zero vector V7.
While the six voltage vectors V1 to V6 are produced, an electric current flowing to a winding of the motor 7 flows to the DC buses 4a and 4b. Thus, it is possible to detect the electric current with the DC current detecting unit 9 and observe the electric current as the DC bus current Idc. On the other hand, while the zero vectors V0 and V7 are produced, it is impossible to observe the electric current as the DC bus current Idc.
FIG. 37 is a table of a relation between eight kinds of basic voltage vectors, switching modes corresponding to the basic voltage vectors, and phase currents that can be observed as a DC bus current Idc. As shown in FIG. 37, it is impossible to observe a phase current in the zero vectors V0 and V7. However, a phase current is observed as “Iu (U-phase current)” in the voltage vector V1, observed as “Iv (V-phase current)” in the voltage vector V2, observed as “−Iw (W-phase current)” in the voltage vector V3, observed as “Iw” in the voltage vector V4, observed as “−Iv” in the voltage vector V5, and observed as “−Iu” in the voltage vector V6.
To rotate the motor 7 smoothly, it is necessary to obtain a magnetic flux corresponding to a desired voltage and a desired frequency. This can be realized by combining the eight kinds of voltage vectors appropriately. FIG. 38 is a diagram for explaining a phase relation between the basic voltage vectors and a relation between the inverter rotation angle and a voltage command vector. In FIG. 38, when an inverter rotation direction is clockwise, the six voltage vectors V1 to V6 are arranged on a phase plane in an order of V1, V3, V2, V6, V4, and V5 clockwise at a phase difference of 60 degrees. The two zero vectors V0 and V7 are shown in an origin position.
In FIG. 38, an inverter rotation angle θ having a direction of the voltage vector V1 (U-phase) as an initial phase gives a phase of a voltage command vector V*. A phase angle between one of the six voltage vectors, which are produced in the inverter rotation direction, and the voltage command vector V* is referred to as a spatial vector rotation angle θ*. Note that an angle range of the spatial vector rotation angle θ* is 0 degree≦θ*<60 degrees.
Production ratios of the respective voltage vectors depend on a percentage modulation that is a ratio of an output voltage to a bus voltage. Production times of the respective voltage vectors depend on the voltage command vector V* and the spatial vector rotation angle θ*. Thus, the phase-current discriminating unit 11 calculates the phase currents Iu, Iv, and Iw from the DC bus current Idc in accordance with the table shown in FIG. 37 while the respective voltage vectors are produced.
Subsequently, the unit 12 for calculating an excitation current and a torque current converts the phase currents Iu, Iv, and Iw calculated by the phase-current discriminating unit 11 into an excitation current Iγ (a γ axis current) and a torque current Iδ (a δ axis current) using, for example, a three-phase to two-phase conversion matrix [C1] indicated by Equation (1) and a rotation matrix [C2] indicated by Equation (2). Note that, in Equation (2), θ indicates an inverter rotation angle, and a rotation direction is clockwise.
                              [                      C            1                    ]                =                                            2              3                                ⁡                      [                                                            1                                                                      -                                          1                      2                                                                                                            -                                          1                      2                                                                                                                    0                                                                                            3                                        2                                                                                        -                                                                  3                                            2                                                                                            ]                                              (        1        )                                          [                      C            2                    ]                =                  [                                                                      cos                  ⁢                                                                          ⁢                  θ                                                                              sin                  ⁢                                                                          ⁢                  θ                                                                                                                          -                    sin                                    ⁢                                                                          ⁢                  θ                                                                              cos                  ⁢                                                                          ⁢                  θ                                                              ]                                    (        2        )            
A coordinate system, on which the unit 12 for calculating an excitation current and a torque current is based, is a γ-δ axis rather than a d-q axis. This point is explained below. An N pole side on a rotor of the motor 7 is set as a d axis and a phase advanced 90 degrees (an electrical angle) in a rotation direction is set as a q axis. When a sensor for detecting a rotor position like a pulse encoder is not used for driving of a synchronous motor, the inverter control unit 2 cannot grasp a d-q axis coordinate of the rotor accurately. Actually, the inverter control unit 2 performs control with a coordinate system shifted by a phase difference Δθ from the d-q axis coordinate system. This coordinate system shifted by the phase difference Δθ is generally referred to as a γ-δ axis coordinate. It is a practice to use this γ-δ axis coordinate. This also applies in this specification.
The voltage-command-vector calculating unit 13 performs various kinds of vector control operation including speed control based on the excitation current Iγ (the γ axis current) and the torque current Iδ (the δ axis current) calculated by the unit 12 for calculating an excitation current and a torque current. The voltage-command-vector calculating unit 13 calculates a magnitude and a phase of the voltage command vector V* used for the next control. This phase angle is the inverter rotation angle θ as described above.
The PWM-signal creating unit 14 produces conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn based on the voltage command vector V* according to various systems described later. Consequently, the PWM-signal generating unit 15 generates three-phase PWM signals Up, Un, Vp, Vn, Wp, Wn, which are driving signals applied to the semiconductor switching elements 5a to 5f, from the conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn and controls the semiconductor switching elements 5a to 5f. As a result, the motor 7 is driven.
As a system for generating a PWM signal in the PWM-signal creating unit 14, conventionally, two systems have been mainly used. One system is a system for generating a PWM signal using four kinds of basic voltage vectors in total, namely, two kinds of basic voltage vectors with a phase difference of 60 degrees and two kinds of zero vectors not having a magnitude that are obtained by switching only one phase of switching states of the two kinds of basic voltage vectors (hereinafter referred to as “three-phase modulation system”). The other system is a system for generating a PWM signal using three kinds of basic voltage vectors in total, namely, two kinds of basic voltage vectors with a phase difference of 60 degrees and one of the two kinds of zero vectors not having a magnitude (hereinafter referred to as “two-phase modulation system”).
Specifically, this is a method of, by decomposing the voltage command vector V* from the voltage-command-vector calculating unit 13 in directions of two basic voltage vectors corresponding thereto, generating production time ratios of the respective basic voltage vectors and calculating conducting times (or non-conducting times) of the respective semiconductor switching elements during one carrier period. This system has problems described below.
A ratio of an output voltage to a DC bus voltage is referred to as a percentage modulation. In the three-phase modulation system or the two-phase modulation system, when the percentage modulation is low, production time ratios of both the two kinds of basic voltage vectors having a magnitude and a phase difference of 60 degrees decreases and a holding time width of a switching mode is narrowed. Even if the percentage modulation is high to some extent, when the voltage command vector V* is close to one of the basic voltage vectors, a production time ratio of the other basic voltage vector distant from the voltage command vector V* decreases and a holding time width of a switching mode is narrowed.
In these two cases, there is a problem in that, in a production section of a basic voltage vector with a short holding time width of a switching mode, since a sufficient DC current detection time cannot be secured and current detection cannot be performed correctly, controllability is deteriorated significantly.
Thus, in recent years, to secure a holding time width of a switching mode in the cases described above, a system for generating a PWM signal with a switching pattern different from the three-phase system and the two-phase system (hereinafter referred to as “extended PWM system”) has been proposed (e.g., patent document 1).
The patent document 1 discloses a three-phase PWM voltage generating circuit that generates a three-phase PWM voltage signal using three kinds of basic voltage vectors in total, namely, two kinds of basic voltage vectors with a phase difference of 120 degrees and a zero vector not having a magnitude that is obtained by switching only one phase of switching states of these basic voltage vectors. The patent document 1 also discloses a three-phase PWM voltage generating circuit that generates a three-phase PWM voltage signal using three-kinds of basic voltage vector having phase differences of 60 degrees, respectively.
In this extended PWM system, the conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn are generated by two methods described below.
(1) As a switching mode during one carrier period, the conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn are generated according to time ratio control for three kinds of vectors in total, namely, two kinds of basic voltage vectors with a phase difference of 120 degrees and a zero vector obtained by switching of only one phase from a switching state of one of the two kinds of basic voltage vectors (this is referred to as “first combination”).
(2) As a switching mode during one carrier period, the conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn are generated according to time ratio control for three kinds of basic voltage vectors having phase differences of 60 degrees, respectively (this is referred to as “second combination”). These methods are explained below with reference to FIGS. 39A to 42.
FIGS. 39A and 39B are diagrams of a relation between the basic voltage vectors used for a first combination in the PWM-signal creating unit shown in FIG. 36 on a phase plane and an example of an order of switching the basic voltage vectors, respectively. FIG. 40 is a timing chart of an example of a logical state (a switching pattern) of a semiconductor switching element on a DC-bus positive-pole side controlled by the first combination.
In the case of the first combination, for example, when an area in which the inverter rotation angle θ is 30 to 90 degrees is considered limitedly, it is possible to generate the conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn by using the basic voltage vectors V1 (0, 0, 1) and V2 (0, 1, 0) with a phase difference of 120 degrees and the zero vector V0 (0, 0, 0) as shown in FIG. 39A and switching the vectors in an order of V0, V2, V0, V1, and V0 as shown in FIG. 39B. A logical state (a switching pattern) of the semiconductor switching elements 5a, 5c, and 5e on a DC bus positive pole side in this case is as shown in FIG. 40. It is seen that a state of output to the motor 7 by the driving signals Wp, Vp, and Up, which are given to the semiconductor switching elements 5a, 5c, and 5e by the PWM-signal generating unit 15, changes in the switching order shown in FIG. 39B.
FIGS. 41A and 41B are diagrams of a relation between the basic voltage vectors used for a second combination in the PWM-signal creating unit shown in FIG. 36 on a phase plane and an example of an order of switching the basic voltage vectors, respectively. FIG. 42 is a timing chart of an example of a logical state (a switching pattern) of a semiconductor switching element on a DC-bus positive-pole side controlled by the second combination.
In the case of the second combination, for example, when an area in which the inverter rotation angle θ is 30 to 90 degrees is considered limitedly, it is possible to generate the conducting time signals Tup, Tun, Tvp, Tvn, Twp, and Twn by using the basic voltage vectors V1 (0, 0, 1), V3 (0, 1, 1), and V2 (0, 1, 0) having phase differences of 60 degrees and switching the vectors in an order of V3, V1, V3, V2, and V3 as shown in FIG. 41B. A logical state (a switching pattern) of the semiconductor switching elements 5a, 5c, and 5e on a DC bus positive pole side in this case is as shown in FIG. 42. It is seen that a state of output to the motor 7 by driving the signals Wp, Vp, and Up, which are given to the semiconductor switching elements 5a, 5c, and 5e by the PWM-signal generating unit 15, changes in the switching order shown in FIG. 41B.
Note that, for example, a patent document 2 discloses a three-phase PWM voltage generating circuit that obtains a sufficient pulse width by contriving pulse width modulation when it is difficult to detect a DC bus voltage. A patent document 3 discloses an inverter or the like that makes it possible to detect an electric current by inserting a carrier wave for one period when it is necessary to detect a DC bus voltage. A patent document 4 discloses a PWM inverter or the like that makes it possible to detect an electric current by preparing a conversion table in advance and setting a pulse width of a DC bus current to a predetermined value or more. A patent document 5 discloses an inverter or the like that makes it possible to perform sampling of a DC bus current even in an inexpensive microcomputer by contriving detection timing of the DC bus current.
Patent Document 1: Japanese Patent Application Laid-open No. 7-298631
Patent Document 2: Japanese Patent No. 3447366
Patent Document 3: Japanese Patent Application Laid-open No. 2003-224982
Patent Document 4: Japanese Patent Application Laid-open No. 2003-209976
Patent Document 5: Japanese Patent Application Laid-open No. 2002-95263